Landforms of the Fluvial System

Objectives

• Describe formation and characteristics of alluvial fans

• Examine landforms formed by fluvial erosion and deposition in river valleys

• Investigate the evolution of river deltas

Introduction

• Fluvial landforms built by:– Deposition– Erosion

Alluvial Fans• Ephemeral streams

emptying from mountain valleys construct alluvial fans

• Alluvial fans are fan-shaped– Formed of alluvial

material– Debris flows in upper

alluvial fans• More common in deserts– But can form in humid

areas

Flow and Deposits on Alluvial Fans

• Flow emptying from valley spread– Velocity decreases,

sedimentation occurs– Unconfined water spreads across

fan in shallow channel

• Deposits are poorly stratified– Thick near mountain– Thin outward from mountains

• Alluvial fans coalesce to form bajadas

• Older parts of fan may develop desert varnish/pavement

Landforms of River Valleys-Meandering Rivers

• Bends in sinuous channel termed meanders– Erosion on outside of

bend– Deposition on inside of

bend—Pt bar– Grow and migrate– Cut-off meanders form

oxbow lakes

Landforms of River Valleys-Meandering Rivers

• Formation of oxbow lakes

Landforms of River Valleys - Braided Rivers

• River with multiple channels separated by sand or gravel bars

• Braiding forms by deposition of bar in mid-channel– Water flows around bar

forming two channels• High sediment load, steep

slopes, variable discharge, loose bank materials may cause braiding

Landforms of River Valleys - Floodplain

• Flat, low-lying ground bordering channel

• Frequently flooded and sediment deposited– Levees form on channel

margin

• Eroded by migrating meandering

• Growing pt. bar rebuilds floodplain

Landforms of River Valleys - Floodplain

• Widening of floodplain and devleopment of large meanders in meander belts

Landforms of River Valleys - Floodplain

• Flooding and levee development

Human Use of Floodplains

• Farming• Development• People do not adjust

well as risk grows• Flood control measures– Artificial levees, dams– Provide mostly artificial

security– Must assume floods will

recur

Terraces• Benches above modern

floodplain– Are older floodplains

• Develop through stream incision of floodplain– Lower floodplain develops– May have paired or mostly

unpaired terraces• May have aggradation before

terrace formation– Increased sediment supply

causes– Reduced sediment supply

then results in incision– Fill terraces produced

Terraces

• Aggradation and terrace development

Deltas

• Triangular shaped deposits where streams empty into oceans or lakes– Coarse sediments deposited at river mouth– Fine sediments carried farther– Distributary channels form transporting water and

sediments

• Delta form depends on:– Discharge and sediment transported– Configuration of seafloor– Strength of currents, waves, tides

Delta Types

The Delta Profile• Consists of bottomset, foreset, and topset beds• Compaction may occur causing subsidence